On this may day thousands are rallying not just for undocumented workers., but immigration reform as well. The group spent the afternoon marching from Union Park to Federal Plaza.

Washington

The Senate’s bipartisan legislation will modestly reduce the flow of foreigners coming into the country in years to come by widening legal immigration channels but cutting off the flow of illegal migration, according to an analysis prepared by the liberal Center for American Progress released Wednesday.

The CAP analysis is a rejoinder to tabulations done by groups opposed to the immigration measure, which have posited that the plan could lead to as many as 30 million more people becoming permanent US residents during the next 10 to 15 years than might have without the reforms.

These calculations are "an attempt to scare the public,” write the authors of the CAP report. "In reality," they argue, "approximately 150,000 fewer people will enter the country each year under the Senate plan.”

Answering the question of how many new immigrants the Senate plan will bring to the US is central to its prospects. Estimates of new Americans not only touch on the cultural fears of some conservatives and economic fears for some job-seekers but also to the hopes of businesses, who want more entrepreneurial and scientific talent. The new CAP figures suggest that liberals are no longer willing to let conservative groups dominate the conversation.

"You have people like [Republican Sen.] Jeff Sesisions and Numbers USA putting out these wild estimates," says Phil Wolgin, a senior immigration analyst with CAP. "Here’s the reality: We’re taking what has been a chaotic system of unauthorized entry on top of legal entry, and we’re moving unauthorized [immigrants] into legal streams."

Not surprisingly, the dueling studies show starkly different assumptions about how to calculate the future flow of immigrants.

For example, CAP does not count the estimated 11.1 million undocumented immigrants currently in the US and who could achieve legal status (and potentially permanent residence and citizenship). It argued that those people are already in the US and thus aren’t “new” to the immigration stream.

Furthermore, they don’t account for the more than 4.5 million prospective immigrants who are waiting in family- and employment-based backlogs. CAP argues that because those people have already been cleared to receive permanent residency – but are waiting for their turn in a sometimes years-long line to get a green card – they don’t count as “new,” either.

The Senate bill would clear the family- and employment-based backlogs before any undocumented person is allowed to achieve permanent residency.

What CAP does do is add the new ways for foreigners to become US residents under the bill (such as a new “merit-based” system) while subtracting reduced or eliminated visa programs (like the diversity visa lottery). That would boost legal migration by some 500,000 new residents per year, CAP estimates.

Then, CAP attempts to account for how the new immigration system – which includes new border-security and employment-verification measures – would affect illegal immigration. Citing a February analysis by two University of Minnesota scholars, CAP argues the US would slash illegal immigration down to just 10 percent of the average 680,000 illegal immigrants who entered the country annually from 2002 to 2009.

“The Senate bill provides a coherent and controlled admissions process that will ensure that those entering the United States come through orderly channels,” according to the analysis.

Other groups question many of CAP's assumptions.

Numbers USA, low-immigration advocates who are battling the bill tooth-and-nail, bases its estimates on both undocumented immigrants currently in the US and prospective immigrants waiting in backlogs that the Senate bill would clear, arguing that these people would formally be allowed to enter the US workforce and compete more directly with American workers.

Those two categories alone will expand US immigration by 15 million during the next decade, Numbers USA says. And that doesn't include the regular stream of 500,000 legal immigrants admitted each year. Add that up over 10 years – and assume that these immigrants will bring over family members – and the number could bulge another 10 million or more.

Indeed, determining how many newcomers might come to the US is devilishly hard – whether you are an advocate for paring back or expanding legal immigration.

Immigration skeptics like Senator Sessions and Numbers USA can't say with much certainty how many family members new US residents would bring with them, for example.

And CAP acknowledges that its baseline of 680,000 annual illegal border-crossers requires nuance. That number excludes the high immigration years from 1999 to 2001 but includes the extremely low years of illegal immigration due to the recent US recession. “Without reform, we would expect future unauthorized immigrant flows to rebound from the lower Great Recession period. This estimate should be taken as a conservative one,” CAP states.

But that number doesn't include how many illegal immigrants left the country, whether through deportation or voluntarily, each year. If that is factored in, the net annual illegal migration from 2002 to 2009 plummets to about 183,000. In 2008 and 2009, it was actually negative.

Moreover, counting illegal immigrants who leave the US raises the question of how to handle net legal migration -- how many permanent residents die or leave the country in a given year, for example. That quickly spirals into another host of challenges and complex calculations.

"In many ways," Mr. Wolgin says, "it's the best calculation we can make."